PlatyPS 2b: migrate Build-PSBuildMarkdown to New-MarkdownCommandHelp

[tablackburn]

Part of #120 (Phase 2 — PlatyPS migration, #105). Blocked by #149.

Scope

Migrate PowerShellBuild/Public/Build-PSBuildMarkdown.ps1 from platyPS 0.14.x to Microsoft.PowerShell.PlatyPS 1.x.

Old	New
New-MarkdownHelp	New-MarkdownCommandHelp
Update-MarkdownHelp	Import-MarkdownCommandHelp | Export-MarkdownCommandHelp

Closes #99 (Build-PSBuildMarkdown tests) — written against the new API, so they land in this PR rather than being written twice.

Watch for

• Parameter surface drift between the old and new cmdlets — anything a consumer sets via $PSBPreference.Docs.* that no longer has an equivalent is a breaking change and needs a migration-guide entry in this PR
• Output shape and file naming: if generated markdown lands in different paths or with a different front-matter schema, consumers with committed docs/ will see churn

Done when

Build-PSBuildMarkdown uses only 1.x cmdlets, #99's tests cover it, CI is green on all legs, and any behavior change has an entry in docs/migration-v0.8-to-v1.0.md.